Solid-state light (SSL) emitting devices, including solid-state light fixtures having light emitting diodes (LEDs) are extremely useful, because they potentially offer long term durability benefits over conventional light fixtures, such as those that utilize incandescent and fluorescent lamps. Due to their long operation (burn) time and low power consumption, solid-state light emitting devices frequently provide a functional cost benefit, even when their initial cost is greater than that of conventional lamps. Rapidly advancing large scale semiconductor manufacturing techniques will enable solid-state light fixtures to be produced at extremely low cost.
In addition to applications such as indicator lights on home and consumer appliances, audio visual equipment, telecommunication devices and automotive instrument markings, LEDs have found considerable application in indoor and outdoor informational displays. For example, LEDs may be incorporated into overhead or wall-mounted lighting fixtures, and may be designed for aesthetic appeal.
With the development of efficient LEDs that emit blue or ultraviolet (UV) light, it has become feasible to produce LEDs that generate white light through wavelength conversion of a portion of the primary emission of the LED to longer wavelengths. This transformation is often described as the “Stokes shift.” Conversion of primary emissions of the LED to longer wavelengths is commonly referred to as down-conversion of the primary emission. Due to metamerism, it is possible to have quite different spectra that, when mixed, appear white. This system for producing white light by combining an unconverted portion of the primary emission with the light of longer wavelength is well known in the art. Other options to create white light with LEDs include mixing two or more colored LEDs in different proportions. For example, it is well known in the art that mixing red, green and blue (RGB) LEDs produces white light. Similarly, mixing RBG and amber (RGBA) LEDs, or RGB and white (RGBW) LEDs, are known to produce white light.
Various methods for manufacturing phosphor LEDs capable of down-converting primary emission have been tested and applied in the art. These methods generally focus on the synthesis of phosphor powders. The blending of phosphor powders is often a cost-effective method to produce a phosphor LED. These traditional methods, however, are generally ineffective for the production of LEDS capable of emitting a multitude of individual wavelengths because it is difficult to control the conversion properties of the blended phosphors. Specific materials and, optionally, dopants may be selected to produce a phosphor having particular light conversion properties, but the resulting powder-based phosphor may generally only be employed to produce a uniform converted light and not a multitude of individual emission wavelengths. Additionally, powder-based phosphors generally require binder materials, such as an organic resin or epoxy, which often have a different refractive index from the phosphor particles. This prevents the phosphor from being optically transparent or translucent and reduces the light extraction efficiency and, hence, the energy efficiency of the phosphor LEDs.
More recently, a number of epitaxial technologies have been developed to produce substrates capable of emitting a multitude of individual wavelengths. Direct emission LEDs based on epitaxial technology requires years of intense development, however, and are generally cost prohibitive. Each emission wavelength produced by the epitaxy substrate requires fine tuning of the device structures, fabrication schemes, optimization of layer compositions, and dopant levels, among other factors, to achieve the particularly desired light conversion properties. The equipment necessary for such manufacturing methods is also costly and difficult to utilize in a stream-lined manner for high volume production.
Similarly, recent technology has sought to manufacture and employ luminescent ceramic phosphors. A luminescent ceramic phosphor may be formed by heating a powder phosphor at high temperature until the surfaces of the phosphor particles begin to soften and a liquid surface layer forms. Interparticle interaction and shrinkage of the particles during sintering produces a rigid agglomerate of particles. Further processing of the sintered predensified ceramic is generally necessary to form a polycrystalline ceramic layer with low residual internal porosity. Unlike a thin film, which optically behaves as a single, large phosphor particle with no optical discontinuities, a luminescent ceramic behaves as tightly packed individual phosphor particles, such that there are small optical discontinuities at the interface between different phosphor particles. Thus, unlike powder-based phosphors, luminescent ceramics are optically almost homogenous and have the same refractive index as the phosphor material forming the luminescent ceramic. This method may be used to produce substrates capable of emitting a multitude of individual wavelengths. Luminescent ceramic phosphors, like powder-based phosphors, are generally incapable of being employed to produce polarized light emission or light propagation.